This invention relates to the field of braking means for sewing machine bobbins, and particularly to means for reducing or eliminating thread spillage in sewing machines.
In lockstitch sewing machines, thread is pulled from the bobbin in small increments at each stitch. Each time this is done, the thread from the bobbin and thread supplied from a spool by way of a needle are locked together and pulled to the proper degree of tautness. In high speed machines, particularly those used in commercial operations, the number of stitches per minute is so great that the bobbin is in almost continuous rotary motion and may easily rotate too far in response to the small rotary impetus it receives at the occurrence of each stitch. This sometimes causes the bobbin to rotate so far forward, because of its angular inertia, that it unwinds an unnecessary amount of thread, which can then easily get tangled up with other parts of the thread or with some of the mechanical components in the region of the bobbin. This excess thread is said to be spilled out, and it is the prevention of such spillage that is a principle object of the present invention.
An additional factor leading to thread spillage is the use of relatively large bobbins which have a correspondingly large inertia. Once such a bobbin is given an incremental rotary impetus, it is more likely to continue to rotate than would a bobbin having a smaller inertia.
A further object of the present invention is to facilitate the use of larger bobbins with higher inertia than has been the case heretofore, and to permit such bobbins to be used in high speed sewing machines.
Various frictional devices have been used heretofore as brakes to limit the excess rotation of bobbins so as to prevent thread spillage. Several types of these devices are based on the concept of the use of a substantially flat spring between the end wall of the bobbin case and the adjacent bobbin flange. By using a spring that is deformed so that it exerts pressure in an axial direction on the inner surface at the end of the bobbin case and on the bobbin flange, a frictional force is produced on the bobbin flange that limits rotation in response to the incremental rotary motion imparted by the operation of removing a short length of thread from the bobbin at the occurrence of each stitch.
If such springs have no means of adjustment other than by being removed temporarily to be bent manually, or by means of a hand tool, it is difficult to cause them to produce the exact, desired braking effect. Instead, such devices tend to produce either too much or too little braking effect.
One way to improve control of the braking effect is to provide a threaded hole in the end wall of the bobbin case and to insert a screw into the threaded hole to press against the resilient member that applies frictional drag to the flange of the bobbin. The amount of pressure on the resilient member can be easily adjusted by turning the screw one way or the other. One such device is shown in U.S. Pat. No. 2,555,658, in which a generally J-shaped spring is attached to the inner part of the bobbin case by means of a small screw that passes through an eye at one end of the J-shaped spring. A second screw that adjusts the frictional drag is inserted in the same end structure of the bobbin case as the screw that holds the J-shaped spring in place.
One of the difficulties of the arrangement just described is that it requires two screws and the wall thickness of the end of the bobbin case has to be great enough at the locations of each of the screws to provide proper engagement between the screw threads and the threaded holes in the wall of the bobbin case.
It is not always possible to allow the end wall of the bobbin case to be sufficiently thick at the necessary locations to insert two separate screws, such as are used in the structure in U.S. Pat. No. 2,555,658, Supra. Accordingly, it is a further object of the present invention to reduce the number of parts while still retaining the ability to provide adjustable brake pressure in a bobbin case and to use a bobbin case having selective reduction of the wall thickness of the end wall at such points as are necessary to accommodate other features of the machine, such as an underbed thread trimmer.